Increase workspace memory area but not data cache area?

Is there any way to increase the amount of memory SQL will use for the workspace memory area but not increase the amount of space for actual data cache?  I'm doing performance testing and scaling the workload by adding more report threads at an 80% buffer cache hit ratio (not what SQL reports, this is logical / (phyical + logical).  I have plenty of memory to add to SQL Server, but if I just increase max memory the amount used for data cache will go up and then my buffer cache hit ratio will increase.  Basically I'm trying to keep the ratio of "disk work" to "cpu work" the same and just add more reports.  It seems like SQL will use up to 60% of memory for workspace...